643 research outputs found
Universal quantum interfaces
To observe or control a quantum system, one must interact with it via an
interface. This letter exhibits simple universal quantum interfaces--quantum
input/output ports consisting of a single two-state system or quantum bit that
interacts with the system to be observed or controlled. It is shown that under
very general conditions the ability to observe and control the quantum bit on
its own implies the ability to observe and control the system itself. The
interface can also be used as a quantum communication channel, and multiple
quantum systems can be connected by interfaces to become an efficient universal
quantum computer. Experimental realizations are proposed, and implications for
controllability, observability, and quantum information processing are
explored.Comment: 4 pages, 3 figures, RevTe
Riemann Surfaces of genus g with an automorphism of order p prime and p>g
The present work completes the classification of the compact Riemann surfaces
of genus g with an analytic automorphism of order p (prime number) and p > g.
More precisely, we construct a parameteriza- tion space for them, we compute
their groups of uniformization and we compute their full automorphism groups.
Also, we give affine equations for special cases and some implications on the
components of the singular locus of the moduli space of smooth curves of genus
g.Comment: 28 pages, 5 figure
Quantum feedback with weak measurements
The problem of feedback control of quantum systems by means of weak
measurements is investigated in detail. When weak measurements are made on a
set of identical quantum systems, the single-system density matrix can be
determined to a high degree of accuracy while affecting each system only
slightly. If this information is fed back into the systems by coherent
operations, the single-system density matrix can be made to undergo an
arbitrary nonlinear dynamics, including for example a dynamics governed by a
nonlinear Schr\"odinger equation. We investigate the implications of such
nonlinear quantum dynamics for various problems in quantum control and quantum
information theory, including quantum computation. The nonlinear dynamics
induced by weak quantum feedback could be used to create a novel form of
quantum chaos in which the time evolution of the single-system wave function
depends sensitively on initial conditions.Comment: 11 pages, TeX, replaced to incorporate suggestions of Asher Pere
Decoherence control in microwave cavities
We present a scheme able to protect the quantum states of a cavity mode
against the decohering effects of photon loss. The scheme preserves quantum
states with a definite parity, and improves previous proposals for decoherence
control in cavities. It is implemented by sending single atoms, one by one,
through the cavity. The atomic state gets first correlated to the photon number
parity. The wrong parity results in an atom in the upper state. The atom in
this state is then used to inject a photon in the mode via adiabatic transfer,
correcting the field parity. By solving numerically the exact master equation
of the system, we show that the protection of simple quantum states could be
experimentally demonstrated using presently available experimental apparatus.Comment: 13 pages, RevTeX, 8 figure
Structure and play: rethinking regulation in the higher education sector
This paper explores possible tactics for academics working within a context of increasing regulation and constraint. One suggested tactic is to move outside of a creativity-conformity binary. Rather than understanding creativity and conformity as separable, where one is seen as excluding the other, the authors consider the potential of examining the relationships between them. The theme of 'structure and play' illustrates the argument. In the first part of the paper, using various examples from art and design - fields generally associated with creativity - the authors explore the interrelatedness of creativity and conformity. For example, how might design styles, which are generally understood as creative outcomes, constrain creativity and lead to conformity within the design field? Is fashion producing creativity or conformity? Conversely, the ways in which conformity provides the conditions for creativity are also examined. For example, the conformity imposed by the state on artists in the former communist bloc contributed to a thriving underground arts movement which challenged conformity and state regulation. Continuing the theme of 'structure and play', the authors recount a story from an Australian university which foregrounds the ongoing renegotiation of power relations in the academy. This account illustrates how programmatic government in a university, with its aim of regulating conduct, can contribute to unanticipated outcomes. The authors propose that a Foucauldian view of distributed power is useful for academics operating in a context of increasing regulation, as it brings into view sites where power might begin to be renegotiated
Glutathione and Gts1p drive beneficial variability in the cadmium resistances of individual yeast cells
Phenotypic heterogeneity among individual cells within isogenic populations is widely documented, but its consequences are not well understood. Here, cell-to-cell variation in the stress resistance of Saccharomyces cerevisiae, particularly to cadmium, was revealed to depend on the antioxidant glutathione. Heterogeneity was decreased strikingly in gsh1 mutants. Furthermore, cells sorted according to differing reduced-glutathione (GSH) contents exhibited differing stress resistances. The vacuolar GSH-conjugate pathway of detoxification was implicated in heterogeneous Cd resistance. Metabolic oscillations (ultradian rhythms) in yeast are known to modulate single-cell redox and GSH status. Gts1p stabilizes these oscillations and was found to be required for heterogeneous Cd and hydrogen-peroxide resistance, through the same pathway as Gsh1p. Expression of GTS1 from a constitutive tet-regulated promoter suppressed oscillations and heterogeneity in GSH content, and resulted in decreased variation in stress resistance. This enabled manipulation of the degree of gene expression noise in cultures. It was shown that cells expressing Gts1p heterogeneously had a competitive advantage over more-homogeneous cell populations (with the same mean Gts1p expression), under continuous and fluctuating stress conditions. The results establish a novel molecular mechanism for single-cell heterogeneity, and demonstrate experimentally fitness advantages that depend on deterministic variation in gene expression within cell populations
Selective quantum evolution of a qubit state due to continuous measurement
We consider a two-level quantum system (qubit) which is continuously measured
by a detector. The information provided by the detector is taken into account
to describe the evolution during a particular realization of measurement
process. We discuss the Bayesian formalism for such ``selective'' evolution of
an individual qubit and apply it to several solid-state setups. In particular,
we show how to suppress the qubit decoherence using continuous measurement and
the feedback loop.Comment: 15 pages (including 9 figures
Measurement of the Michel Parameters in Leptonic Tau Decays
The Michel parameters of the leptonic tau decays are measured using the OPAL
detector at LEP. The Michel parameters are extracted from the energy spectra of
the charged decay leptons and from their energy-energy correlations. A new
method involving a global likelihood fit of Monte Carlo generated events with
complete detector simulation and background treatment has been applied to the
data recorded at center-of-mass energies close to sqrt(s) = M(Z) corresponding
to an integrated luminosity of 155 pb-1 during the years 1990 to 1995. If e-mu
universality is assumed and inferring the tau polarization from neutral current
data, the measured Michel parameters are extracted. Limits on non-standard
coupling constants and on the masses of new gauge bosons are obtained. The
results are in agreement with the V-A prediction of the Standard Model.Comment: 32 pages, LaTeX, 9 eps figures included, submitted to the European
Physical Journal
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